This invention resides in the field of respiration systems and controls therefor, and is especially adapted for use with volume-limited ventilators in which a measured volume of gas is delivered to a patient during each inhalation phase of a positive pressure breathing system. It is more particularly directed to a method for reliably ascertaining maximum and minimum lung pressure during each breathing cycle.
Respiration apparatus for positive pressure breathing therapy is in common use, and it has been the usual practive to determine the volume of gas delivery and to control it in accordance with measurements made at the respiration machine. However, the total compliance of the system is a significant factor and one that does not remain constant at all times. Thus, the volume of gas actually delivered to the patient may vary relative to the desired or selected amount.
The effectiveness of patient ventilation depends on an exchange of ample tidal volume of gas in the lungs during each breathing cycle. This is related to the difference between minimum and maximum pressure in the lungs, the maximum pressure occurring at the end of the inspiration phase and the minimum at or near the end of the expiration phase. Should respiration apparatus be adjusted to cycle at too fast a rate, then the lungs may not have time to force out the gas. This could result in the residual or minimum lung pressure climbing to an unacceptable level. Thus, it is highly desirable from the standpoint of adjusting cycle rate, as well as for diagnostic purposes, to know the maximum and minimum lung pressures.
Many improvements have been made over the years in the use of equipment and correction factors to reduce the variation in volume delivery to a low level, and the results are generally quite satisfactory. However, the tidal volume desired for infants is far less then for adults and may be as low as 5 percent or less of the adult requirements. Consequently, the effects of any variation of the system and of any differences that exist in pressure between system and the lungs are correspondingly magnified. Accordingly, there is a need for a supply and control system which makes it possible to monitor the lung pressure and especially the minimum and maximum pressures during each cycle.